Rickettsiae are obligate intracellular bacterial pathogens that cause diseases such as spotted fever and typhus. We study the model spotted fever group (SFG) species Rickettsia parkeri, which causes human rickettsiosis, is experimentally tractable, and has a mouse model of pathogenesis, making it ideal for revealing molecular mechanisms of SFG Rickettsia infection and virulence. Similar to pathogens such as Listeria monocytogenes, SFG Rickettsia require the host actin cytoskeleton to invade, move within, and spread between host cells. However, Rickettsia are distinct from other pathogens in their mechanisms of manipulating actin. Therefore, studying Rickettsia will advance our understanding of how and why pathogens exploit diverse actin-polymerizing mechanisms, and will reveal new features of actin function and regulation in uninfected cells. Our past work has made important contributions to understanding how Rickettsia mobilize actin. We uncovered a core set of host cytoskeletal proteins that mediate invasion, and separately revealed host proteins uniquely important for Rickettsia intracellular motility. Moreover, we discovered that Rickettsia are distinct from other pathogens in using two sequential phases of motility - an early phase that depends on the bacterial RickA protein, and a late phase that depends on the bacterial Sca2 protein. Despite these advances, how Rickettsia mobilize the cytoskeleton during infection is poorly understood. Using a panel of recently isolated transposon mutants of R. parkeri, we will test the overall hypothesis that Rickettsia deploy proteins including Sca2, their type IV secretion system (T4SS), the putative T4SS effectors and actin-associated proteins RickA and Sca4, as well as ankyrin-repeat proteins Ank1 and Ank2, to mobilize actin during host cell invasion, intracellular motility and cell-cell spread. We will answer three questions. First, how do bacterial proteins promote actin assembly during invasion? Second, how and why do Rickettsia use two actin assembly factors to enable distinct phases of intracellular motility? Third, how do bacterial proteins contribute to cell-cell spread? More specifically, in Aim 1, we will test whether and how Rickettsia use Sca2, the T4SS, and the putative T4SS effectors Sca4, RickA and Ank2 in invasion and virulence, and determine if these factors are T4SS effectors. In Aim 2, we will test whether early and late motility promote avoidance of bacterial killing by autophagy, determine when and how a transition between early and late motility occurs, and examine the molecular mechanisms by which RickA and Sca2 mediate motility and virulence. In Aim 3, we will determine the role of early and late motility in cell-cell spread, and examine the importance of the T4SS, RickA, Sca4, Ank1 and Ank2 in modulating actin and membrane dynamics at distinct stages of spread. These studies will reveal new mechanisms of host-pathogen interactions, and may result in new ways of diagnosing and treating infections.